The present invention relates to the use of compounds that block the α9α10 subtype of the nicotinic acetylcholine receptor (nAChR) for treating pain, such as neuropathic pain and inflammatory pain, and inflammatory disorders, such as rheumatic diseases. The present invention also relates to screening compounds to identify analgesic agents that block the α9α10 subtype of the nAChR.
The publications and other materials used herein to illuminate the background of the invention, and in particular, cases to provide additional details respecting the practice, are incorporated by reference, and for convenience are referenced in the following text by author and date and are listed alphabetically by author in the appended bibliography.
Predatory marine snails in the genus Conus have venoms that are rich in neuropharmacologically active peptides (Armishaw and Alewood, 2005; Wang and Chi, 2004; Livett et al., 2004; Lewis, 2004; Terlau and Olivera, 2004). There are approximately 500 species in Conus, and among those that have been examined so far, a conserved feature is the presence of α-conotoxins in their venom. These are highly disulfide cross-linked peptides with the disulfide scaffold of C1-C3 and C2-C4. Due to high sequence variability of their non-cysteine residues α-conotoxins are extremely diverse and each Conus species has a unique complement of α-conotoxins. α-Conotoxins are synthesized as large precursors, and the mature toxin is generated by a proteolytic cleavage toward the C-terminus of the precursor. In contrast to the variable inter-cysteine sequences of the mature toxins, the precursors and the genes encoding them are quite conserved both among α-conotoxins in a given Conus species and from species to species. α-Conotoxins have generally been shown to be nicotinic acetylcholine receptor (nAChR) antagonists (McIntosh et al., 1999; Janes, 2005; Dutton and Craik, 2001; Arias and Blanton, 2000).
nAChRs are a group of acetylcholine gated ion channels that are part of the ligand gated ion channel superfamily (Karlin, 2002; Gotti and Clementi, 2004). They are pentamers of transmembrane subunits surrounding a central ion conducting channel. Many different subunits have been identified, and most fall into two main subfamilies (the α subunits and the β subunits). The subunits can associate in various combinations in the receptor pentamers, leading to a diverse family of receptor subtypes. Most of the subtypes contain subunits from both the α and β subunit families, e.g., the human adult muscle subtype contains two α1 subunits and a β1 subunit (in addition to a δ and an ε subunit) and the α3β2 subtype is composed of α3 and β2 subunits. nAChRs that are composed of only α subunits are the α7 and α9 subtypes (homopentamers) and the α9α10 subtype (an all α heteropentamer). Phylogenetic analysis shows that the α7, α9, and α10 subunits are more closely related to each other than they are to other nAChR subunits (Le Novere et al., 2002; Sgard et al., 2002).
The α9 and α10 nAChR subunits are expressed in diverse tissues. In the inner ear α9α10 nAChRs mediate synaptic transmission between efferent olivocochlear fibers and cochlear hair cells (Sgard et al., 2002; Elgoyhen et al., 1994; Elgoyhen et al., 2001). The α9 and α10 subunits are also found in dorsal root ganglion neurons (Harberger et al., 2004; Lips et al., 2002), lymphocytes (Peng et al., 2004), skin keratinocytes (Arredondo et al., 2002; Nguyen et al., 2000; Kurzen et al., 2004), and the pars tuberalis of the pituitary (Sgard et al., 2002; Elgoyhen et al., 1994; Elgoyhen et al., 2001).
There are drugs used in the treatment of pain, which are known in the literature and to the skilled artisan. See, for example, The Merck Manual of Diagnosis and Therapy, 17th Edition (1999). However, there is a demand for more active analgesic agents with diminished side effects and toxicity and which are non-addictive. The ideal analgesic would reduce the awareness of pain, produce analgesia over a wide range of pain types, act satisfactorily whether given orally or parenterally, produce minimal or no side effects, be free from tendency to produce tolerance and drug dependence.